The molecular processes which direct cell-cell communication between the embryo and the uterus during implantation are not well defined. However, the spatio-temporal expression of several growth factors suggests their importance in an intricate cell-signaling network. The goal of this proposal is to determine the role of growth factors of the neuregulin family in cell signaling between the blastocyst and the surrounding uterus during implantation. Neuregulins are unique members of the EGF growth factor family that induce the formation of various heterodimers of the erbB/EGF receptors. Three different neuregulin genes have been identified: NRG1, NRG2, and NRG3. Dr. Reese and his collaborators in the Dey laboratory previously demonstrated that neu differentiation factor (NDF), an NRG1 isoform, is expressed in the uterus solely at the site of implantation and requires the presence of an activated blastocyst. Preliminary results also show that NRG3 and certain other NRG1 isoforms are expressed in the uterus and pre-implantation blastocyst, but that NRG2 expression is absent. Thus, neuregulin-mediated cell signaling is isoform-specific and restricted to the time of implantation. The hypothesis to be tested is that selective members of the NRG family of growth factors are involved in embryo-uterine cell signaling, and that these ligands act in a coordinated fashion with particular erbB receptors. To address this question, three specific aims are proposed: 1) determine the temporal and cell-specific expression of representative members of the neuregulin family of growth factors in the mouse embryo and uterus during the peri-implantation period; 2) examine whether the blastocyst's state of activity under the influence of ovarian estrogen and progesterone alters the regulation of neuregulin expression; and 3) evaluate ligand-receptor specificity in neuregulin-mediated cell signaling between the embryo and uterus. To accomplish these aims, the investigators will carry out quantitative RT-PCR, Northern and in-situ hybridization, receptor phosphorylation, and cell adhesion studies. Results obtained from these studies are likely to provide important information regarding the basic mechanisms of fertility regulation in women.